I build stuff with just the "-j" option (unlimited jobs) to Make all the time. Had a 75 load average the other day. Also Rust servo compiles which aren't GCC but certainly load down the machine.
Processor errata, chipset errata, bugs in the Linux kernel, bugs in the compilers, and application issues. There is so much opportunity for strange behavior. It takes a long time to work it all out.
Intel has had people contributing patches for a very long time. AMD had better get a team on it, if they don't already.
Well, I would say that ASRock is not the best motherboard brand around. Try Asus or MSI next time.
Having said that, they're one of the smaller manufacturers, and it wouldn't surprise me if they're still in the process of getting on top of some of the issues that inevitably come up with a new platform.
It also wouldn't surprise me if GP got a lemon. It can happen with any manufacturer.
As it is, though, I'm a very happy customer of theirs.